An increasing number of photovoltaic (PV) systems, which combine a photovoltaic module and a power conditioner, are now connected to the grid (or utility grid) and to loads (or electric devices).
Power conditioners (power conditioning systems, or PCSs) for PV systems typically use maximum power point tracking (MPPT) control. A common PV system including a PCS with MPPT control can extract maximum power from its photovoltaic module. However, when the input power of the PCS varies greatly, all the power generated by the photovoltaic module may not be usable.
The PV system may incorporate a rechargeable battery that is charged and discharged to control the input power of the PCS to a value near a target value (refer to, for example, Patent Literature 1). However, the control for charging and discharging the rechargeable battery incorporated in the PV system can interfere with the MPPT control performed by the PCS. Also, the MPPT control performed by the PCS can differ depending on each individual manufacturer of the PV system, and its details are not open. A rechargeable battery controller that can control the input power of the PCS in an existing PV system to a value near a target value without affecting MPPT control remains undeveloped.